For instance, last year Facebook introduced a version of its
Credits payment platform for mobile developers of Facebook apps.
But
Apple won't let developers use Credits on iPhone or iPad apps
-- they have to route payments through Apple's App Store instead,
giving Apple (rather than Facebook) the 30% cut of each
transaction.

Here's how Facebook lays out the risk in its S-1:

We are dependent on the interoperability of Facebook with popular
mobile operating systems that we do not control, such as Android and iOS, and any changes in such systems that
degrade our products’ functionality or give preferential
treatment to competitive products could adversely affect Facebook
usage on mobile devices. Additionally, in order to deliver high
quality mobile products, it is important that our products work
well with a range of mobile technologies, systems, networks, and
standards that we do not control. We may not be successful in
developing relationships with key participants in the mobile
industry or in developing products that operate effectively with
these technologies, systems, networks, or standards.